Snap-acting photoelectric relay



p 1964 R. w. NAYLOR SNAP-ACTING PHOTOELECTRIC RELAY Filed Oct. 26, 1961 INVENTOR. ROBERT W. NAYLOR ATTORNEY United States Patent Office 3,151,280 Patented Sept. 29, 1964 3,151,280 SNAP-ACTHNG PHOTOELECTRIC RELAY Robert W. Naylor, Weston, Ontario, Canada, assignor to CTS of Canada, Ltd, Ontario, Canada, a corporation of Ontario, Canada Filed Oct. 26, 1961, Ser. No. 147,879 6 Claims. (Cl. 317-124) This invention relates to photoelectric relays generally and in particular to photoelectric relays which are con trolled by a light source, such as sunlight, which changes its intensity very slowly.

One of the most common uses of a photoelectric relay is in the control of street lights, patio lights, etc. where it is desirable for the lights to be turned oil during the daylight hours and on during the night. Since the number of hours of daylight per twenty-four hour period changes with the seasons, the best method of automatically controlling these lights is by using a switch controlled by the intensity of the sunlight. In this manner, when the sun is furnishing suificient light, the switch is open and the light is oil. Then when the sunlight diminishes to some predetermined level, the switch isclosed and the light comes on.

These switches generally comprise a single pole, singlethrow switch which is spring biased toward the closed position and which uses an electromagnet to overcome the spring and to move the switch to open position. The amount of current flowing through the electromagnet is controlled by a photoconductive element so that when the sun in shining, the resistance of the element is low causing sufiicient current to flow through the coil of the electromagnet to keep the electromagnet energized and'open the switch. Conversely, when the sun is not shining, the photoconductive cellwill pass little or no current keeping the electromagnet de-energized and the switch closed.

This prior art system is completely satisfactory with one exception. During the early morning hours when the intensity of the daylight is slowly rising, the current passing through the photoconductive cell will be slowly increasing, with a resultant slow increase in the force being exerted by the electromagnet on the switch. This force willincrease until the switch blade is on the verge of opening. At this instant, the force being exerted by the electromagnet is just sufiicient to balance the force being exerted by the spring causing the switch to chatter; that is, the movable contactor will move back and forth makingv and breaking contact with the stationary contact of the switch. If the current through the relay is pulsating, this making and breaking of the switch causes pitting of the contacts of the switch and the subsequent early failure of the contacts. If the current through the relay is not pulsating, the contacts will part so gradually that a high resistance connection will be created. This results in heating of the contacts and usually a subsequent welding together thereof.

It is, therefore, a principal object of this invention to provide a photoelectric switch which will snap open without any tendency to chatter or form'a high resistance contact when the light intensity reaches a predetermined amount. It is an additional object of this invention to provide an auxiliary light source which will cause a rapid decrease in the resistivity of the photoconductive element and a subsequent rapid. opening of the switch when the ambient light intensity reaches a predetermined value.

It is a further object of this invention to provide means whereby the auxiliary light source functions only to assist in the rapid opening of the switch after which time it becomes inoperative.

Additional objects and advantages of this invention will be obvious to those skilled in the art when taken in connection with the detailed description set out below and the attached drawing in which a schematic diagram of the circuitry involved in this snap-acting photoelectric switch is illustrated.

In the drawing, the principal circuit comprises the conductors 1i and 12. These conductors connect the load which, as illustrated, comprises street lights 13 and 14 to source of alternating current indicated symbolically at 15. Arranged in series with the conductor 11 is a single pole, double-throw switch comprising a movable contactor 16, a stationary contact 1.7, and a stationary contact 18. As shown, the movable contactor 16 is in engagement with the stationary contact 17. In this position, current is allowed to flow from the alternator 15 to the street lights 13 and 14, through the conductors 11 and 12. Resiliently holding the movable contact 16 in this position is the coil spring 19.

Arranged to exert a force on the movable contactor 16 opposite to the spring I?) is an electromagnet 20, consisting of the conventional coil and soft iron core. To provide the necessary unidirectional current to the electromagnet, it is connected across the output of the rectifier 22. A shorted turn in the form of a copper ring or slug should be placed around the soft iron core to help smooth the effect of the pulsating current. Of course, if DC. current was being provided to the lights 13 and 14, neither this shaded pole technique nor the rectifier would be necessary.

Connected in series with the input of the rectifier is a photoconductive cell 21. In this application of the invention, the photoconductive cell is of the type which becomes electrically conductive in the presence of light rays, such as rays in the visible light spectrum and adjacent infra-red and ultra-violet regions and which is highly resistive to current flow in the absence of light.

The remainder of the circuit illustrated is made up of a fixed resistor 28, an electric lamp 23 which, for the purposes of this description is a neon lamp but need not be limited thereto, and a resistor 24. Resistor 28 and neon lamp 23 are connected in series between the power line 11 and the junction of the photocell 21 and the rectifier 22. When the electromagnet 20 is energized, resistor 24 is placed in parallel with the neon lamp 23.

In operation, when it is nighttime and no light of suflicient strength is available to lower the resistance of the photoconductive cell 21, the circuit is in the condition illustrated in the drawing. The movable contractor 16 is held in contact with the stationary contact 17 by the spring 19 and alternating current is supplied to the load which is, in this case, street lights 13 and 14.

As it starts getting light in the morning, the resistance of the photoconductive cell 21 begins to decrease slowly. This allows a small amount of current to begin to flow through the lead 26, through the rectifier 22, and through the coil of the electromagnet 20. This current flow creates 'an electromagnetic force which tends to pull the movable contactor 16 away from the stationary contact 17. By properly designing the strength of the spring 19 and the voltage drop across the various components of the circuit, it is predetermined that the electric glow lamp 23 will strike before the electromagnet will have pulled the movable contactor 16 away from the stationary contact 17. In other words, the circuit is so designed that before the force of the electromagnet is able to overcome the spring 19, enough light will have fallen on the photoconductive cell 21 to. allow sufiicient Voltage'to appear across the electric lamp '23 to cause it to glow and illuminate the photocell. When this occurs, the instantaneous flash of light which is in view of the photoconductive cell 21 suddenly reduces the resistance of the photoconductive cell 21 to the point where current can flow freely through the conductor 26 and the rectifier 22 into the coil of the electromagnet 20. An increasingly strong electromagnetic force is suddenly produced and the movable contactor 16 is rapidly moved out of contact with the stationary contact 17 and into engagement with the stationary contact 18.

At this point, it is necessary to reduce the voltage across the lamp 23 to below that necessary for its continued operation. Otherwise, the lamp would keep the photoconductive cell in a condition of low resistance and the switch would never close again.

To do this, the resistive value of the fixed resistors 24 and 28 is such that when the movable contactor fit? is moved into contact with the stationary contact 18 by the electromagnet 20, the voltage drop across the lamp 23 drops below that necessary for its operation. The photoconductive cell will now be in condition to change its resistance in accordance with the ambient light and to close the switch and turn on the lights when darkness again approaches.

Even though the lamp becomes inoperative as soon as the switch is opened, there will be sufficient current flowing through the coil of the electromagnet at this point to hold the movable contactor away from the stationary contact 17 since the length of the magnetic path in the relay has been reduced and much fewer ampere turns are required to hold the switch open.

The invention has been explained in connection with a photoconductive relay used to turn lights on or on in response to the ambient light conditions. It is, of course, possible to produce many other embodiments without departing from the inven ive concept herein disclosed, and it is desired that only such limitations be imposed on the appended claims as are stated therein, or required by the prior art.

The invention claimed is:

1. In a control system, the combination of an electrical device to be controlled, a relay, a photoconductive cell, connected in series with the relay and subject to a varying light intensity, for controlling the energization of the relay, the relay and the cell being connected across a source of voltage, and an auxiliary light source connected in series with the photoconductive cell and in parallel with the relay and physically arranged to irradiate the photoconductive cell whereby upon increasing the intensity of the light irradiating against the photoconductive cell the auxiliary light becomes conductive and instantaneously increases the amount of light irradiating against the photoconductive cell thereby increasing the current through the relay for rapidly opening a pair of contacts to disconnect the electrical device from the source of voltage.

2. In a control system, the combination of an electrical device to be controlled, means connected across a source of voltage for connecting and disconnecting the electrical device to be controlled to the source of voltage, a photoconductive cell connected in series with the above-mom tioned means, and auxiliary light means connected in series with the photoconductive cell and in parallel with the first-mentioned means whereby as the voltage across the auxiliary light means is increased by an increase in the intensity of light irradiating against the photoconductive cell the auxiliarylight means becomes conductive to increase substantially the intensity of light irradiating against the photoconductive cell thereby increasing rapidly the current flowing through the first-mentioned means for rapidly disconnecting the electrical device from the source of voltage.

3. The control system of claim 2, wherein a resistor is connected in parallel with the auxiliary light means to reduce the voltage across the auxiliary light means and a make the light means'nonconductive after the first-mentioned means disconnects the electrical device from the source of voltage.

4. In a control system, the combination of an electrical device to be controlled, means connected in parallel with a source of voltage for connecting and disconnecting a circuit connecting the electrical device to the source of voltage, a photoconductive cell, associated with the above-mentioned means and subject to a varying light intensity, for controlling the energization of the above mentioned means, an auxiliary light connected across the source of voltage in series with the photoconductive cell and physically arranged to irradiate the photoconductive cell, a first resistor connected in series with the auxiliary light, and a second resistor adapted to be connected in parallel with the auxiliary light after the above-mentioned means disconnects the electrical device from the source or": voltage.

5. In a control system, the combination of an electrical device to be controlled, a circuit comprising a full Wave rectifier and a photoconductive cell in series with the rectifier connected across a source of voltage, a circuit comprising a neon bulb and a first resistor in series with the neon bulb connected across the rectifier and in series with the photoconductive cell, a relay connected across the output terminals of the rectifier, the neon bulb being physically arranged to irradiate the photoconductive cell, a switch engageable by the relay for connecting and disconnecting the electrical device to the source of voltage, and a second resistor adapted to be connected across the neon bulb after the relay rapidly opens the switch to disconnect the electrical device from the source of voltage to make the neon bulb nonconductive.

6. In a control system, the combination of an electrical device to be controlled, a full wave rectifier having one side connected to one side of a source of voltage, a photoconductive cell having one side connected to the other side of the full wave rectifier, the other side of the photoconductive cell being connected to the other side of the source of voltage, an auxiliary li ht having one side connected to the one side of the source or" voltage, a first resistor having one side connected to the other side of the auxiliary light, the other side of the first resistor being connected to the one side of the photoconductive cell, the auxiliary light and the first resistor being in series with the photoconductive cell and in parallel with the full wave rectifier, a switch having a movable contact connected to the one side of the source of voltage, a first stationary contact connected to the electrical device to be controlled, a second resistor, a second stationary contact connected to one side of the second resistor, the other side of the second resistor being connected to the other side of the auxiliary light, and a relay connected across the output terminals of the full wave rectifier whereby upon increasing the intensity of light irradiating against the photoconductive cell the auxiliary light becomes conductive causing a sudden increase in the intenrity of light upon the photoconductive cell to increase substantially the current through the relay to disconnect the movable contact from the first contact thereby disconnecting the electrical device to be controlled from the source of voltage and cansing the movable contact to engage the second stationary contact to connect the second resistor in parallel with the auxiliary light to make the auxiliary light nonconductive.

References Iited in the file of this patent UNITED STATES PATENTS 

1. IN A CONTROL SYSTEM, THE COMBINATION OF AN ELECTRICAL DEVICE TO BE CONTROLLED, A RELAY, A PHOTOCONDUCTIVE CELL, CONNECTED IN SERIES WITH THE RELAY AND SUBJECT TO A VARYING LIGHT INTENSITY, FOR CONTROLLING THE ENERGIZATION OF THE RELAY, THE RELAY AND THE CELL BEING CONNECTED ACROSS A SOURCE OF VOLTAGE, AND AN AUXILIARY LIGHT SOURCE CONNECTED IN SERIES WITH THE PHOTOCONDUCTIVE CELL AND IN PARALLEL WITH THE RELAY AND PHYSICALLY ARRANGED TO IRRADIATE THE PHOTOCONDUCTIVE CELL WHEREBY UPON INCREASING THE INTENSITY OF THE LIGHT IRRADIATING AGAINST THE PHOTOCONDUCTIVE CELL THE AUXILIARY LIGHT BECOMES CONDUCTIVE AND INSTANTANEOUSLY INCREASES THE AMOUNT OF LIGHT IRRADIATING AGAINST THE PHOTOCONDUCTIVE CELL THEREBY INCREASING THE CURRENT THROUGH THE RELAY FOR RAPIDLY OPENING A PAIR OF CONTACTS TO DISCONNECT THE ELECTRICAL DEVICE FROM THE SOURCE OF VOLTAGE. 